It's not very hard to explain. In English class you are doing an in-depth look at all aspects of the English language, rather than just how to speak and write it. You read great literature, write five-paragraph essays, analyze sentence structure, as well as many other things. However, if you are taking a lower level Spanish class, or any language for that matter, you are only learning things such as tenses, sentence structure, and basic vocabulary. I mean, until you get into upper tier courses you don't do anything nearly equivalent to a high school english class, aside from reading a book or two. In short, rather than do all of the crap I listed above, someone who already speaks a language and is taking that same language course really just has to spout off basic sentences, like "Hi, my name's Joe" or "Could you direct me to the supermarket, I need some apples," things you almost every child could say and write by the first or second grade. The only kids who run into problems are the ones that learned to speak a language when they were young, but did not learn how to write it.